The invention relates to multiplexers with high noise immunity, suitable for use in industrial control applications.
Industrial control applications often require input from tens or hundreds of remote sensors. Wiring these devices to a central control panel can become complex and costly. Multiplexing reduces system complexity by transmitting all signals on a common communication media. In addition, a multiplexed system reduces installation time, lowers maintenance, and provides system flexibility.
The present invention provides a 3-wire, fully isolated multiplex system. The high noise immunity afforded by the invention is particularly desirable in electrically noisy environments such as industrial control applications.
The invention features a master-controlled system. A master station provides all power, clock, and synchronization signals to remote stations. A 3-wire shielded communication cable links the master station and the remote stations. The master station and each remote station is fully isolated from the communication cable. Each remote station may be at a different ground potential with no effect on system operation. Signal transmission is in digital form, and analog quantities may be digitized and transmitted on adjacent channels. Transmitter and receiver type remote stations may communicate directly, and real time applications are possible. A simple yet high security data encoding technique is used. Remote receiver stations feature optional built-in error detection and selectable fail-safe modes.
The basic multiplexer of the invention includes a master station, a plurality of remote stations, and a cable comprising three wires linking the master station and the remote stations. Two wires of the cable are power lines L1 and L2 for powering, clocking and synchronizing the remote stations from the master station. The remote stations do not require local power supplies and do not require local clocks. Isolation is achieved by a combination of transformer and optical techniques. AC power, clock, and sync signals are all transformer coupled. Data is on the third wire and is optically coupled. In all cases, no ohmic connection exists between the cable and any of the master station or remote stations.
The signaling system for data encoding provides high data security without wasted channel time slot space. The encoding format is efficient, yet highly secure. Data is transmitted by a combination of active and passive encoding. The data line is switchingly connected to and then disconnected from one of the power lines in accordance with a given signaling format.
In the preferred embodiment, the power lines carry AC power, and the switching system comprises switchingly connecting the AC power to the data line at designated times. The AC signal also provides clocking. Each multiplexed channel consists of one or more complete cycles of the AC clock. In the single cycle embodiment one logic state is encoded by closing a switch between the AC signal and the data line during the positive half cycle of the clock and opening the switch during the negative half cycle. The opposite logic state is encoded by opening the switch during the positive half and closing it during the negative half. Both active (switch closed) and passive (switch open) states are required to establish valid data, i.e. during one of the half cycles there must be current flow on the data line, either positive or negative, and during the other half cycle there must be a null state on the data line, with no current flow. In remote receiver stations, detector circuitry checks both the position within a cycle and the polarity of each data pulse. Using this method, errors and failed equipment are easily detected.
In one particular embodiment, the system provides 32 channels scanned in 1.5 milliseconds. A 25 kilohertz AC square wave provides power and clocking. The system enables line voltage of 30 volts RMS, whereby Class 2 wiring methods may be used. Signal current of 30 milliamps is afforded.